Although plastic optical fibers comprising a plastic core and a plastic clad are poorer than glass optical fibers in transmission characteristics, the former are better than the latter in flexibility. Therefore, the plastic optical fibers have such advantages that they are easy to be bent and difficult to be folded, have a good shock resistance and are easily worked at their end faces. Accordingly, in case of short transmission distances of less than 10 m, the plastic optical fibers are often used because transmission loss is negligible.
In the prior art, the plastic optical fibers comprise a fiber body of a plastic core and a plastic clad and a cover of low density polyethylene, for example, provided on the fiber body. However, such plastic optical fibers have such drawbacks that they are easily burned, poor in both thermal and chemical resistance so that they are easily damaged by gasoline in case they are used as internal wiring for automobiles.
Therefore, it is proposed that polyvinyl chloride (referred to as PVC hereinafter), which is non-flammable material, is used as a cover for the fiber body instead of low density polyethylene. However, such optical fibers have a drawback that plasticizers included in the PVC migrate into the fiber body so that transmission characteristics are made poor as described by Japanese Utility Model Application Laying Open No. 74403/1982.
Alternatively, there are proposed by Japanese Utility Model Application Publication No. 7362/1983 plastic optical fibers having dual covers of a first layer of saponified copolymer of ethylene and vinyl acetate and a second layer of non-flammable material. However, such optical fibers require two covering steps so as to be expensive to produce. Although they can be produced by one step, it requires two extruders, which causes the apparatus to be complicated.